Known inboard boat drives comprise an underwater propulsion and steering unit which is equipped with one or two propellers and can be swivelled about a vertical axis. The swivellability of the propulsion vector created by the propellers enables a steering effect for the boat to be attained. The swivelling is carried out using a steering shaft which is controlled by a steering unit. The particular steering angle (the angle of the propulsion vector relative to the midship axis) is measured using an angular sensor, wherein the swivel motion of the steering shaft is transferred via gears to the angular sensor. The disadvantage of the known measurement devices is that the rotational motion is transferred with play, and therefore the angle that is displayed is inaccurate.